Freeze 2 cups cubed watermelon until solid. Meanwhile, bring 3/4 cup each sugar and water to a boil, stirring to dissolve the sugar. Transfer to a pitcher and refrigerate until cold. Puree 4 more cups cubed watermelon in a blender; strain into the pitcher. Stir in 1 cup lime juice (from about 9 limes) and 2 cups cold seltzer. Add the frozen watermelon and some lime slices.